If it's something that's bothering me at the time that I'm pretty sure will bother me in the future, I would say something--especially if it's about something they have a tendancy to do, which in that case they may benefit from being aware of it.
I had a friend in college who was LOUD. On top of that, she didn't understand the fine are of subtlety--if we tried to point someone out quietly, she would be the one to yell "YOU MEAN THE GUY IN THE RED SHIRT AND BLACK HAT WITH THE DREADS SITTING NEXT TO THE BIG DUDE WEARING LIME GREEN?!"
The first couple of times we laughed it off, but after a while it was like, yo, wtf. Finally I asked her if she's noticed how loud her speaking voice is, and had she ever considered that people are whispering and being on the down low for a reason, but she's the only one giving away the position?
Turns out I wasn't the only person who had told her that, and she subsequently decided to have some testing done. Didn't help with her lack of subtlety, but at least she made an effort to lower her volume.